A focus on margins irrespective of industry leads to incorrect analysis. For example, look at Comcast, a company that truly operates as a monopoly/duopoly in most of its markets. It has an operating margin of 7.6%.

On the other hand, Exxon Mobil has an operating margin of nearly 20% despite selling an undifferentiated commodity in a market with many well-capitalized competitors. (They are not the only one: ConocoPhillips also sports an operating margin in the range of 20%.)

The particulars of a market often drive margins more than does the competition.